Techniques for carrying out various kinds of processing such as piercing, cutting, and welding by using laser light have conventionally been known. For example, Japanese Patent Publication No. HEI 5-42336 discloses a method of bonding members with laser. In this method, on a first thermoplastic resin member having a property of absorbing laser light of a YAG laser, a second thermoplastic resin member having a property of transmitting the laser light therethrough is overlaid. Then, the YAG laser irradiates the first thermoplastic resin member with the laser light by way of the second thermoplastic rein member, so as to heat and melt the first thermoplastic resin member, thereby welding the first and second thermoplastic resin members to each other.
When carrying out processing with laser light, it is important that the temperature in the processing area be controlled in order to prevent the processing from becoming defective. For detecting the temperature of the processing area, a radiation thermometer, which uses the light thermally radiating from the processing area, or the like is employed. For example, Japanese Patent Application Laid-Open No. HEI 5-261576 discloses a heat processing apparatus which regulates the surface temperature of a subject to be processed when welding the subject with laser light. In this heat processing apparatus, the light thermally radiating from the surface of the subject to be processed is divided into a plurality of light components, which are then transmitted through various filters adapted to transmit respective wavelength light components therethrough. A surface temperature is detected from the intensity ratio of various wavelength light components transmitted through the filters, and the surface temperature of the subject to be processed is controlled according to thus detected surface temperature.